This invention relates to a propeller driving system for a marine propulsion unit and more particularly to an improved arrangement for taking the thrust from a propulsion shaft of a forward, reverse transmission of a marine propulsion unit.
Conventionally, the propulsion shaft such as a propeller shaft of a marine propulsion unit such as an outboard motor or the outboard drive portion of an inboard/outboard drive includes a forward, neutral, reverse transmission so that the propulsion shaft can be selectively driven in forward and reverse directions. Such transmissions normally use a bevel gear reversing transmission that includes a driving bevel gear that is coupled to the drive shaft of the outboard drive unit and which drives a pair of counterrotating bevel gears that can be selectively coupled to the propulsion shaft for driving the propulsion shaft in forward or reverse directions.
Because of the forward and reverse drive of the propulsion shaft, it is necessary to insure that the lower unit has thrust bearings that operate to take the thrust in both forward and reverse directions. In one type of arrangement, the driven bevel gears are rotatably supported on a gear mounting shaft that is axially separate from but splined to the propulsion shaft. Alternatively, arrangements have been provided in which both the forward and reverse driven gears are journaled on a single propulsion shaft and adapted to be clutched to the shaft for driving it in forward or reverse directions. Either arrangement, however, presents some problems in connection with the taking of the thrust adequately in both directions and supporting all of the elements. For example, with the first mentioned type of mechanism employing a separate gear mounting shaft, the gear mounting shaft cannot be long enough to secure sufficient axial support length for each gear so as to be loaded stably and also to secure a sufficient span between the gear mounting shaft bearings to avoid shaft inclination caused by play in the bearings. Such shaft inclination can have adverse effects on the gear contact of the transmission. Also, the splines are apt to be worn by the action of shifting from forward to reverse and vice versa.
With the other type of mounting arrangement wherein the gears are mounted on a single propulsion shaft, it is difficult to provide accurate dimensional accuracy between both thrust transmitting portions so that the resulting play will cause longitudinal movement of the propeller shaft, reduced durability of the bearings and so forth. In addition, such arrangements can introduce noise to the system which is a forerunner of wear.
It is, therefore, a principal object of this invention to provide an improved propeller driving system for a marine propulsion unit having an improved bearing and thrust arrangement.
It is a further object of this invention to provide an improved arrangement for taking the thrust from a propeller shaft of the type that mounts both the forward and reverse driving gears and wherein the thrust bearings are adequately preloaded.
It is a further object of the invention to provide an improved bearing arrangement for a propulsion shaft of a marine drive lower unit wherein forward and reverse thrusts are taken and the bearings are preloaded by means of a spring.